Various methods and systems have been used to attempt to control boundary layer flow in aerodynamic systems. These include passive methods for boundary layer control including static roughness, passive, compliant surfaces, and active methods such as flaps, rudders and ailerons. In particular, several research initiatives have sought to utilize morphing concepts for flow control purposes. The most notable large-scale agency-sponsored efforts include the DARPA Micro Adaptive Flow Control (MAFC) program and micro-aero adaptive control in the NASA Morphing project, both of which have exploited smart structures and advanced discrete MEMs actuator concepts that require low actuation energy to target control of large-scale aerodynamic flows and multi-point adaptability in aircraft operations.
There is a rich technical literature that describes laminar flow, turbulent flow, transitions between the two flow regimes, and the interaction of flow regimes with various surface conditions, such as surface roughness.
Classical results for turbulent boundary layers, such as Townsend's hypothesis that the outer-scaling layer is not affected by the roughness other than through the friction velocity, and the logarithmic scaling of the mean velocity with the effective roughness amplitude, δ, as the scaling lengthscale, hold only when k is small compared to the boundary layer thickness, δ, i.e., k/δ<<1, which is not always satisfied in lower Reynolds number experiments. The latter scaling also only holds when the flow has reached equilibrium.
In laminar flows, roughness is known to enhance linear receptivity to external disturbances, leading to increases in the disturbance growth rates and reductions in the critical Reynolds number for transition to turbulence.
In practical airfoil flows, under certain conditions in-flight wing icing may lead to the generation of a temporary static roughness distribution, with detrimental, or at least unplanned, implications for the aerodynamic performance of the vehicle.
Previous patents in the field of flow control include U.S. Pat. No. 4,516,747, issued May 14, 1985 to Lurz entitled “Method and apparatus for controlling the boundary layer flow over the surface of a body,” U.S. Pat. No. 5,961,080 issued Oct. 5, 1999 to Sinha, entitled “System for efficient control of flow separation using a driven flexible wall,” and an International Application published under the PCT as International Publication No. WO 2006/040532 A1, published Apr. 20, 2006, in the names of Morrison, J. F., Dearing S. S., Arthur, G. G., McKeon, B. J and Cui, Z., entitled “Intelligent Fluid Flow Surfaces.” U.S. Pat. No. 4,516,747 describes a system for boundary layer control by damping of laminar instability or turbulent velocity fluctuations using sensors and vibration transmitters. U.S. Pat. No. 5,961,080 describes a flexible wall that can be used either to drive or sense an external flow by means of sub-surface capacitive transducers. International Publication No. WO 2006/040532 A1 describes distributed “active”, or time-dependent, dimples fabricated from electrostrictive polymer with surface-deposited electrodes. In particular, International Publication No. WO 2006/040532 A1 teaches that the use of dimples is “much more effective than surface roughness.” International Publication No. WO 2006/040532 A1 describes designs and manufacturing methods that are specifically intended to achieve a deflection of a continuous sheet to provide depressions or dimples in the continuous sheet in preference to protrusions.
A number of problems in applying previous flow control methods and systems have been observed. One drawback to using passive devices for flow control lies in degraded aerodynamic performance in flow regimes away from those where the control is useful. Some drawbacks in using some active devices can be the complexity of a control system, and the need for expendables such as gases or liquids that are consumed, which add weight, and which limit the duration of operation of the system by virtue of being available in only a finite amount.
There is a need for improved aerodynamic control surface systems and methods.